In the manufacture, assembly and adjustment of many industrial processes and applications there is often a need to obtain accurate and precise measurements of geometric surface characteristics, e.g. to determine the flatness of the surface, determine the degree of inclination at different areas, etc. To this end, various techniques have been developed over the years.
One such technique is to user laser beams or other electromagnetic radiation, and measure the point of impact on sensors, such as position sensing devices (PSD) or the like. Such methods are e.g. disclosed in U.S. Pat. No. 7,403,294 and DE 10 2008 035 480.
Another approach has been to use photogrammetry, wherein targets are arranged on the subject to be measured, and photographs are taken from different angles. By processing the images, e.g. by means of triangulation, the geometric position of the targets may be calculated, and be used to determine the geometric surface characteristics of the surface of the object. Such solutions are e.g. discussed in U.S. Pat. No. 6,826,299 and U.S. Pat. No. 6,990,215.
However, some objects are more difficult to measure precisely than others. It has been found that flanges arranged on tubular objects, and in particular very large tubular objects, are very cumbersome and costly to measure with conventional techniques, and that the measurement results suffer from errors making the measurement data less reliable. This is e.g. the case when measuring wind power generation towers. These towers are made by tubular, slightly conical sections, being assembled together at the site of use. The connections between the sections are made by connecting flanges together with bolts or the like. The towers are often very high, and have to endure very strong lateral forces. Further, the flanges need to have a well-defined radial inclination, in order to ensure that the outer periphery, in case of inwardly oriented flanges, at all places remain in full contact, and to ensure that there is not too much load on the bolts. Inadequate peripheral contact would create instability, and a great risk of bolt failure when subject to external forces. To this end, to ensure that the flatness and radial inclination of the flanges are always within correct and acceptable tolerances is of utmost importance. However, these tower sections are often very large, often having a cross-sectional diameter of several meters, and frequently even between 5 and 10 meters. It would be preferred to measure the flanges of these sections already during manufacturing. However, measurement of these flanges with presently known techniques is both, as a practical matter, tedious and costly, and the measurement data is often not sufficiently accurate and reliable.
To this end, it has been proposed in U.S. Pat. No. 8,240,044, to measure the towers during assembly, and to use specifically made shims to adjust the measured unevenness. However, measurement during assembly is both costly and difficult.
There is therefore a need for an improved method for measuring geometric surface characteristics of a flange surface positioned at an end of a tubular object, and in particular for very large tubular objects.